What helped me to lift those sinking legs nearer the surface in Superman glide was to experiment with the position the outstretched arms were at in the glide.
When my arms were just under the surface my legs rapidly made their way to the bottom of the pool. By trial & error I found that if the elbows of my outstretched arms were below my head I was able to keep those heavy legs up & to glide for longer. Say that the water surface is 3 0 clock, I found that pointing my hands to about 4.30 made a big difference.
Give it a try, it worked for me.